The present invention relates generally to fuel cell systems and particularly, to a counter-flow heat exchanger mechanization to transfer reformate energy to steam and air for efficient operation of hydrocarbon fuel processors. The present invention may be used in small-scale, highly integrated fuel cell systems, such as those used in automobiles or homes.
Two issues contribute to the limited use of hydrogen gas in powering small-scale, highly integrated fuel cell systems, such as those used in automobiles or homes. Firstly, hydrogen gas (H2) has a low volumetric energy density compared to conventional hydrocarbons, meaning that an equivalent amount of energy stored as hydrogen will take up more volume than the same amount of energy stored as a conventional hydrocarbon. This is a concern given the limit of fuel storage available to automobiles and homes. Secondly, there is presently no widespread national hydrogen supply infrastructure that could support a large number of fuel cell powered automobiles or homes.
However, an attractive source of hydrogen for such fuel cell systems is contained in the molecular structure of various hydrocarbon and alcohol fuels. Current small scale, highly integrated fuel cell systems use a reformer or fuel processor to break down the molecules of a primary fuel to produce a hydrogen-rich gas stream capable of powering the fuel cells. Generally, for the efficient operation of such hydrocarbon fuel processors, the primary reactor must have a high reformate exhaust temperature, typically from about 700° C. to about 750° C. Lowering the reformate exhaust temperatures below the above-mentioned range results in significant methane formation, which decreases net hydrogen production. Accordingly, in order to achieve and maintain this high reformate exhaust temperature, the reactor feed streams (i.e., fuel, steam, and air) are preheated. Preheating also minimizes the amount of air needed, which in turn maximizes efficiency.
With current designs, the primary reactor-out reformate and, if available, the tail gas combustor exhaust are the only two sources of waste heat hot enough to be useful in preheating the steam and air inputs into the primary reactor. To extract such waste heat, prior art fuel processors use a heat exchanger inline with the primary reactor-out reformate to preheat the steam and air inputs. Typically, this heat exchanger is placed inline between the primary reactor and water gas shift reactor. The reformate continues in the same flow direction through these three units, as well as other downstream units.
To minimize pressure drop and ensure a uniform velocity distribution throughout the various system reactors and heat exchanger, the face area of the heat exchanger in contact with the reformate flow is typically designed to match that of the reactors. Additionally, to minimize mass and volume, as well as keep the overall length of the fuel processor small for packaging, the distance of the reformate flow through the heat exchanger needs to be kept small, typically <50% of the diagonal dimension of the face area. This low aspect ratio has resulted in the use of a cross-flow heat exchanger design in such prior art fuel cell systems.
The use of cross-flow heat exchangers, however, is not without consequences. In particular, the use of cross-flow heat exchangers in such prior art fuel cell systems has limited the maximum heat exchanger effectiveness and has increased the risk of having a non-uniform reformate temperature at its outlet. Additionally, steam and air exiting such inline, cross-flow heat exchangers typically are carried in a pipe to the primary reactor inlet, where fuel is introduced and mixed. Significant heat loss to the ambient can occur along this pipe section, even if substantially insulated. To minimize such heat loss, prior art solutions involve keeping the pipe connecting the heat exchanger to the inlet of the primary reactor of the fuel processor as small as possible in diameter. However, it has been found that reducing the pipe diameter results in poor distribution between the fuel and the resulting high-velocity, small entry-area stream of steam and air. Non-uniform distribution of fuel, air, and steam leads to portions of the primary reactor running richer than desired resulting in cooler temperatures, higher methane formation, lower primary reactor efficiency, and others portions running leaner than desired resulting in higher temperatures leading to primary reactor catalyst degradation.
With regard to heating the steam and air input to the primary reactor with tail gas combustor exhaust, one problem involves the need for an additional heat exchanger to preheat the combustor feed streams (e.g., stack anode exhaust, and either stack cathode exhaust or combustor air). This additional heat exchanger increases the fuel processor mass, volume, cost, pressure drop, complexity, and startup time. In addition, because the tail gas combustor is the last component in the fuel cell system, there is the potential for transient lags between primary reactor-based requirements and combustor-based responses. Finally, using tail gas combustor exhaust heat usually requires that the tail gas combustor be located close to the primary reactor to minimize heat loss in the primary reactor-in air pipe, which in small scale, highly integrated fuel cell systems may not always be the preferred packaging.
Accordingly, the present inventors have recognized a need for improvements in heat exchanger mechanization to transfer reformate energy to steam and air for efficient operation of hydrocarbon fuel processors.